Electroplating



Patented Nov. 12, 1940 ELECTROPLATING Christian John Wernlund, NorthTonawanda, N. Y.. assignor to E. Ldn Pont de Nemours & Company,Wilmington, DcL, a corporation of Delaware No Drawing. ApplicationDecember 24, 1937,

' Serial No. 181,617

6 Claims. (01. 204-40) This invention relates to electroplating nickeland more particularly to the electrodeposition of multiple layers ofmetal including a layer of nickel. I

This application is a continuation in part of my co-pending applicationSerial Number 120,315, filed January 12, 1937, issued November 28, 1939,as U. S. Patent 2,181,773.

Nickel plating is utilized'for coating a large variety of meta-larticles. While the nickel is often deposited directly on the basemetal, it is general practice first to deposit a layer of copper oryellow brass on the base metal and then to electroplate nickel on thisintermediate layer. An important use for nickel plating is to coat metalsurfaces composed ofzinc or a zinc alloy containing at least by weightof zinc, e. g., zinc alloy die castings. In these utilizations, thenickel either may be the final decorative coating or it may serve as anundercoating for chromium or other metal. In the electroplating of zincalloy die castings as practiced heretofore, three methods have beenemployed. In one method, the nickel is electroplated directly onto thedie casting. In this method, it is necessary to add sodium sulphate orother conducting salt to the nickel solution in fairly large quantitiesand the at a high current density, then follow with electrodeposition atthe normal current density.

Satisfactory nickel deposits by this method require careful control onthe part of the operator. Also, unless the nickel deposit isexceptionally dense and relatively thick, the zinc or zinc alloyunderneath the nickel tends to corrode causing the formation of blistersand eventual peeling of the deposit. p

A second method which has been proposed is to electroplate yellow brassas an undercoating for the nickel deposit. While this method usuallygives satisfactory results if properly done, the electroplating of thebrass is difficult and requires exceptionally careful control to obtainuniform results. If the bath temperature and current density are notmaintained substantially constant, the composition of theelectrodeposited brass will vary considerably. Moreover, it is necessaryto use a low current density which results in a slow deposition of thebrass.

The third method consists of electroplating copper as the undercoat forthe nickel electrodeposit. The difliculty with this method is that intime the copper undercoating often tends to alloy with and diffuse intothe zinc alloy casting,

so that finally the benefit of the undercoating disappears and thenickel is subject to the same blistering action whic'h occurs whennickel is deposited directly on the zinc alloy. To prevent thisoccurrence, it is necessary to plate a relatively thick layer of copper,adding to the manufacturing cost. A further-disadvantage is that unlessthe nickel deposit is exceptionally dense and non-porous, corrosionproducts from the copper undercoat appear on the surface of the mckledarticle, causing green discoloration.

An object of the present invention is to provide an improved method forproducing nickel plated articles, which method is free of thedisadvantages enumerated above. A further object is to provide animproved method for producing nickel plated articles made of zinc orzinc alloy, for example, nickel plated zinc alloy die castings. Otherobjects will be apparent from the following description.

The above objectsjmay be attained in accordance with the presentinvention by first electroplating the article to ibe nickel plated witha layer of a zinc copper alloy which contains from 19 to 31% of copperand then electroplating nickel on to the zinc copper alloy from anacidic solution of a nickel salt. By this method I am able to producenickel plated zinc die castings or other zinc articles which will standfor long periods of time under corrosive conditions of the atmospherewithout blistering or peeling of the nickel coating and without anytendency for copper staining. I have found that theelectro platedcoating of zinc copper alloy containing not more than 31% of copperundergoes no reaction with the underlying zinc; that is to say, there isno tendency for thecopper in the alloy coating to diffuse, into the zincalloydie casting. I have further found that the zinc copper alloyundercoating is suiliciently resistant to corrosion so that there issubstantially no tendency for blistering and peeling of the nickelcoating to occur. Also, any corrosive products from the zinc copperalloy which may be formed in small amounts apparently are white orcolorless, for no green stains appear upon the nickel coatin when thelatter is subjected to strenuous corrosion tests.

Another advantage of my nickel copper undercoating is that it readilymay be brought to the proper degree of brightness required forelectroplating a bright nickel deposit thereon. In fact, if desired, thezinc copper alloy'may be directly alloy surface.

plated in a bright condition so that substantially no bufling. isrequired. I have also found that the zinc copper alloy is considerablyharder than either pure copper or yellow brass and that consequently inthe bufiing operation there is much less danger or removing too much ofthe alloy to expose base metal at corners and edges of the article.Also, there'is no difliculty in obtaining a dense adherentelectrodeposit of nickel on the zinc alloy undercoating and the controlpf the nickel plating solution is easier than, for example, when nickelis plated directly on to a zinc or zinc In electroplating my zinc alloyundercoating, I prefer to utilize the'method described and claimed in myabove mentioned Patent No. 2,181,773. In this method a cyanide bathcontaining the double alkali metal cyanides of copper and zinc and analkaline material such as caustic soda is used.

The following is an example of a solution suitable for this purpose: Perliter The composition of the zinc copper alloy may be controlled asdesired withinthe range 19 to 31% of copper by varying the amount ofcopper cyanide in the electrolyte and the temperature of theelectroplating solution. In general, the amount of zinc cyanide in theelectrolyte should be equal to three to four times that of the coppercyanide. It is also preferred to maintain the concentration ratio ofzinc cyanide to sodium hydroxide to about one to one; however, thisratio may be varied between the approximate limits of .66 to 2.0; thatis, the "weight of caustic soda may be equal to from 50 to 150% of theweight of the zinc cyanide. By thus maintaining the concentration ratioof zinc cyanide to caustic soda as above stated, the composition of thezinc. copper alloy will remain substantially constant throughout longperiods of operation under varying conditions of current density. Theelectrolyte may be operated at any desired temperature. from roomtemperature, e. g. 20 C., up to the boiling point of the solution. Igenerally prefer to operate the bathat a temperature of about 45 C. Itshould be noted that the composition of the electrodeposit will varysomewhat as the temperature is changed, the copper content of theelectrodeposit increasing as the temperature is raised. Hence, byvarying the temperature of operation, the composition of theelectrodeposit may be varied as desired.

It should be noted that the present invention is not restricted to theabove method for electroplating the zinc copper alloy which has beendescribed merely by way of example. Various solutions containing copperor zinc cyanides or other salts which are suitable for electroplatingcopper zinc alloys may be used to practice my invention. In using suchsolutions it is, of course, necessary to regulate the respective amountsof zinc and copper salts in the electrolyte and to regulate otherfactors such as temperature and current density so as to obtain anelectrodeposit containing from 19 to 31% copper. The thickness of theelectrodeposit of the zinc copper alloy may be varied considerably withgood results and may be as low as 0.0001 inch in thickness. In general,I prefer to electroplate a layer of the zinc copper alloy about 0.0003to 0.0005 inch thick. Much thicker coatings may be used if desired butordinarily there is little or no advantage in a thickness of more than0.0005 inch thick.

After the zinc copper alloy has'been plated on the zinc alloy gr otherbase metal, if it has not been plated in a bright condition, it shouldbe buffed before plating it with nickel. The buffing operation issimilar to bufling zinc prior to electroplating; that is, it isdesirable to use a greaseless bufling compound to avoid the formation ofinsoluble zinc soaps on the surface of the metal.

If desired, bufilng may be eliminated by electroplating the copper zincalloy in a bright condition according to the method described in theabove mentioned co-pending patent application. In order to plate themetal in'the bright condition it is necessary to use a cyanide bath and,prior to electroplating; to subject the solution to a purificationtreatment which removes substantially all traces of heavy metalimpurities such as soluble lead and cadmium coz'npoun'ds'which may bepresent therein. This may be done by addin a soluble sulphide or othersoluble sulphur compound which will cause a precipitation of such heavymetals as sulphides. Alternatively, the heavy metal impurities may beremoved by adding an emcient reducing agent, for example, zinc dust.After precipitation of the impurities, the

.bath may be allowed to stand or may be filtered.

' ing it in an acidic oxidizing solution, for example,

a dilutesolution of nitric acid, e. g. 0.5-5%, or a hydrogen peroxidesolution acidified with sulphuric acid. The resulting surface is smoothand lustrous and usually is practically identical with a well buffedsurface. k I

In accordance with my invention the above described zinc copper alloycoating. containing 19-31% of copper is electroplated with a nickelplate of any desired thickness, e. g. 0.0003 to 0.001 inch thick. It isessential that the nickel be electroplated from an acidic solution. Anyof the various well known acidic nickel plating baths may be utilizedfor my purpose. The following formula is an example of a well knownnickel plating solution which gives satisfactory results:

Ounces per gallon NiSO4.6H2O 16 NiChS g 4 Boric acid 4 Y 4 tained at apH of 5 to 6. .The acidity may be adjusted by the addition ofhydrochloric or sulphuric acid in the required amount. If the solutionis inadvertently made too acid, it may be readjusted to the desired pHvalue by the addiponderantly of zinc, although castings may be made frompure zinc. For example, certain zinc die castings which weresuccessfully coated with nickel by my method were found to contain about95% by weight of zinc and about 5% by weight of aluminum and less than1% by weight of cop per. In this description and in the appended claims,I use the term zinc die castings to include alloy castings containing90% or more by weight of zinc and also castings of pure zinc.

It is further understood that my invention is not restricted to coatingcast zinc articles, but is applicable to the coating of any metalsurface composed of zinc or a zinc alloy containing at least 90% byweight of zinc. For example, zinccoated iron and steel articles, such asgalvanized iron or articles having an electroplated zinc surfacelikewise may be coatedwith nickel by my method, with excellent results.

I claim:

1. An article of manufacture comprising a metallic base composedpreponderantly of zinc having a multiple, electrodeposited metal coatingcomprising a layer of white brass at least 0.0001 inch in thicknesscontaining 19 to 31% by.

electroplated onto said white brass layer.

2. An article of manufacture comprising a zinc alloy base containing atleast 90% by weight of zinc, having a multiple, electrodeposited metalcoating comprising a layer of white brass at least 0.0001 inch inthickness containing 19 to 31% 40 by weight of copper, the remainderbeing zinc,

electroplated onto said base and a layer of nickel electroplated ontosaid white brass layer and a coating of chromium on said nickel layer.

3. An article of manufacture comprising a zinc die casting having acoating at least 0.0001 inch in thickness of white brass containing 19to 31% by weight of copper, the remainder being zinc, electroplated ontothe surface of said die casting and having an adherent coating of nickelelectroplated onto said white brass coating.

4. The process for coating a metal surface composed preponderantly ofzinc comprising electroplating said surface with a white brass coatingat least 0.0001 inch in thickness containing 19-31% byweight of copperthe remainder being zinc and thereafter electroplating nickel onto thewhite brass coating from an acidic nickel plating solution.

5. The process for coating a metal surface containingnot less than byweight of zinc, comprising electroplating said surface with a whitebrass coating at least 0.0001 inch in thickness containing 19 to 31% byweight of copper the remainder being zinc and thereafter electroplatingnickel onto the white brass coating from an acidic nickel platingsolution, while maintaining the acidity of said solution within the pHrange of 4 to 6.5.

6. The process for coating a zinc die casting comprising electroplatingthe surface of said casting with a white brass coating at least 0.0001inch in thickness containing 19 to 31% by weight of copper the remainderbeing zinc from a solution containing zinc and copper cyanides, andthereafter electroplating nickel onto the white brass coating from anacidic nickel sulfate solution, while maintaining the acidity of saidsolution within the pH range of 4 to 6.5.

CHRISTIAN JOHN WERNLU'ND.

